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docs/ARCHITECTURE: mention src/fundamental/ and add more details

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Zbigniew Jędrzejewski-Szmek 2022-06-17 15:14:44 +02:00
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@ -9,8 +9,8 @@ SPDX-License-Identifier: LGPL-2.1-or-later
## Code Map
This section will attempt to provide a high-level overview of the various
components of the systemd repository.
This document provides a high-level overview of the various components of the
systemd repository.
## Source Code
@ -21,30 +21,60 @@ names are self-explanatory.
### Shared Code
You might wonder what kind of common code belongs in `src/shared/` and what
belongs in `src/basic/`. The split is like this: anything that is used to
implement the public shared objects we provide (`sd-bus`, `sd-login`,
`sd-id128`, `nss-systemd`, `nss-mymachines`, `nss-resolve`, `nss-myhostname`,
`pam_systemd`), must be located in `src/basic` (those objects are not allowed
to link to `libsystemd-shared.so`). Conversely, anything which is shared
between multiple components and does not need to be in `src/basic/`, should be
in `src/shared/`.
The code that is shared between components is split into a few directories,
each with a different purpose:
- `src/basic/` and `src/fundamental/` — those directories contain code
primitives that are used by all other code. `src/fundamental/` is stricter,
because it used for EFI and user-space code, while `src/basic/` is only used
for user-space code. The code in `src/fundamental/` cannot depend on any
other code in the tree, and `src/basic/` can depend only on itself and
`src/fundamental/`. For user-space, a static library is built from this code
and linked statically in various places.
- `src/libsystemd/` implements the `libsystemd.so` shared library (also
available as static `libsystemd.a`). This code may use anything in
`src/basic/` or `src/fundamental/`.
- `src/shared/` provides various utilities and code shared between other
components that is exposed as the `libsystemd-shared-<nnn>.so` shared library.
The other subdirectories implement individual components. They may depend only
on `src/fundamental/` + `src/basic/`, or also on `src/libsystemd/`, or also on
`src/shared/`.
You might wonder what kind of code belongs where. In general, the rule is that
code should linked as few times as possible, ideally only once. Thus code that
is used by "higher-level" components (e.g. our binaries which are linked to
`libsystemd-shared-<nnn>.so`), would go to a subdirectory specific to that
component if it is only used there. If the code is to be shared between
components, it'd go to `src/shared/`. Shared code that that is used by multiple
components that do not link to `libsystemd-shared-<nnn>.so` may live either in
`src/libsystemd/`, `src/basic/`, or `src/fundamental/`. Any code that is used
only for EFI goes under `src/boot/efi/`, and `src/fundamental/` if is shared
with non-EFI compoenents.
To summarize:
`src/fundamental/`
- may be used by all code in the tree
- may not use any code outside of `src/fundamental/`
`src/basic/`
- may be used by all code in the tree
- may not use any code outside of `src/basic/`
- may not use any code outside of `src/fundamental/` and `src/basic/`
`src/libsystemd/`
- may be used by all code in the tree, except for code in `src/basic/`
- may not use any code outside of `src/basic/`, `src/libsystemd/`
- may be used by all code in the tree that links to `libsystem.so`
- may not use any code outside of `src/fundamental/`, `src/basic/`, and
`src/libsystemd/`
`src/shared/`
- may be used by all code in the tree, except for code in `src/basic/`,
`src/libsystemd/`, `src/nss-*`, `src/login/pam_systemd.*`, and files under
`src/journal/` that end up in `libjournal-client.a` convenience library.
- may not use any code outside of `src/basic/`, `src/libsystemd/`, `src/shared/`
`src/libsystemd/`, `src/nss-*`, `src/login/pam_systemd.*`, and files under
`src/journal/` that end up in `libjournal-client.a` convenience library.
- may not use any code outside of `src/fundamental/`, `src/basic/`,
`src/libsystemd/`, `src/shared/`
### PID 1